There has always existed within the fishing industry a need to determine the presence (or absence) of fish over a wide area of fishing grounds in a rapid and expeditious manner. Such determination would allow fishing boats to proceed directly to areas having the highest potential for catch. Presently, the time consuming practice of searching small areas one at a time on a trial and error basis is relied upon to locate fish. Schools of fish can move from an unsearched position to a location that has already been fished--giving the false impression that no fish are present at the moment.
Some of these problems are solved by the devices described in my U.S. Pat. No. 4,596,085 issued June 24, 1986 and in my U.S. Pat. No. 4,649,744 issued Mar. 17, 1987. However, further improvements are possible.